NASA Study Reveals Hidden Stage of Arctic Freeze

As autumn gives way to winter across the Arctic, the ground doesn't immediately freeze solid. Instead, soils often linger near the freezing point for days or even weeks in a phase called the zero curtain. In a new NASA-led study, researchers produced high-resolution maps of the Arctic zero curtain. Understanding the duration, intensity and extent of this phenomenon should help scientists better predict how thawing permafrost, or ground that is frozen for an extended length of time, could affect Earth's climate.

The zero curtain works much like a glass of ice water. The water remains at the freezing point because incoming heat melts the ice rather than raising the temperature. The same effect occurs within the Arctic soil during spring. In autumn, the reverse happens: as water freezes, it releases heat that holds the ground near the freezing point before it can cool further.

The near-freezing conditions allow microbes in the soil to remain active longer, releasing carbon dioxide and methane into the atmosphere. Arctic permafrost stores an estimated 1.9 trillion tons (1.7 trillion metric tons) of organic carbon, which is nearly twice the amount currently in Earth's atmosphere. As frozen ground thaws, more of that carbon could be released as microbes take advantage of the expanding zero curtain's moist conditions.

In the Aug. 18 issue of Scientific Reports, researchers described an artificial intelligence framework called GeoCryoAI that combines satellite observations and model outputs with field measurements dating back to 1891 to create the first detailed maps of zero-curtain conditions across the region around the Arctic. The maps show that spring thaw generally produces much longer zero-curtain periods than autumn freeze-up and that regions with higher moisture levels experiencing longer zero-curtain periods.

The team designed GeoCryoAI to work with data from US-India NISAR (NASA-ISRO Synthetic Aperture Radar) mission, which could help scientists observe how frozen landscapes respond to a changing climate and improve forecasts of future greenhouse gas emissions.